As is well known to those skilled in the art, it is possible to separate mixtures of liquids, typified by mixtures of water and organic liquids such as aqueous solutions of ethylene glycol or isopropanol, by various techniques including adsorption or distillation. These conventional processes, particularly distillation, are however, characterized by high capital cost. In the case of distillation for example, the process requires expensive distillation towers, heaters, heat exchangers (reboilers, condensers, etc.), together with a substantial amount of auxiliary equipment typified by pumps, collection vessels, vacuum generating equipment, etc.
Such operations are also characterized by high operating costs principally costs of heating and cooling--plus pumping, etc.
Furthermore the properties of the materials being separated, as is evidenced by the distillation curves, may be such that a large number of plates may be required, etc. When the material forms an azeotrope with water, additional problems may be present which for example, would require that separation be effected in a series of steps (e.g. as in two towers) or by addition of extraneous materials to the system.
There are also comparable problems which are unique to adsorption systems.
It has been found to be possible to utilize membrane systems to separate mixtures of miscible liquids by pervaporation. In this process, the charge liquid is brought into contact with a membrane film; and one component of the charge liquid preferentially permeates the membrane. The permeate is then removed as a vapor from the downstream side of the film--typically by sweeping with a carrier gas or by reducing the pressure below the saturated vapor pressure of the permeating species.
Illustrative membranes which have been employed in prior art techniques include those set forth in the following table:
TABLE ______________________________________ Separating Layer References ______________________________________ Nafion brand of Cabasso and Liu perfluorosulfonic acid J. Memb. Sci. 24, 101 (1985) Sulfonated polyethylene Cabasso, Korngold & Liu J. Pol. Sc: Letters, 23, 57 (1985) Fluorinated polyether USP 4,526,948 or Carboxylic Acid fluorides to Dupont as assignee of Resnickto Selemion AMV Wentzlaff brand of Asahi Glass Boddeker & Hattanbach cross-linked styrene J. Memb. Sci. 22,333 butadiene (with quaternary (1985) ammonium residues on a polyvinyl chloride backing) Cellulose triacetate Wentzlaff, Boddeker & Hattanback, J. Memb. Sci. 22, 333 (1985) Polyacrylonitrile Neel, Aptel & Clement Desalination 53, 297 (1985) Crosslinked Eur. Patent 0 096 Polyvinyl Alcohol 339 to GFT as assignee of Bruschke Poly(maleimide- Yoshikawa et al acrylonitrile) J. Pol. Sci. 22, 2159 (1984) Dextrine- Chem. Econ. Eng. isophorone diisocyanate Rev., 17, 34 (1985) ______________________________________
The cost effectiveness of a membrane is determined by the selectivity and productivity. Of the membranes commercially available, an illustrative polyvinyl alcohol membrane of high performance is that disclosed in European patent 0 096 339 A2 of GFT as assignee of Bruschke--published Dec. 21, 1983.
European Patent 0 096 339 A2 to GFT as assignee of Bruschke discloses, as cross-linking agents, diacids (typified by maleic acid or fumaric acid); dihalogen compounds (typified by dichloroacetone or 1,3-dichloroisopropanol); aldehydes, including dialdehydes, typified by formaldehyde. These membranes are said to be particularly effective for dehydration of aqueous solutions of ethanol or isopropanol.
This reference discloses separation of water from alcohols, ethers, ketones, aldehydes, or acids by use of composite membranes. Specifically the composite includes (i) a backing typically about 120 microns in thickness, on which is positioned (ii) a microporous support layer of a polysulfone or a polyacrylonitrile of about 50 microns thickness, on which is positioned (iii) a separating layer of cross-linked polyvinyl alcohol about 2 microns in thickness.
Polyvinyl alcohol may be cross-linked by use of difunctional agents which react with the hydroxyl group of the polyvinyl alcohol. Typical cross-linking agent may include dialdehydes (which yield acetal linkages), diacids or diacid halides (which yield ester linkages), dihalogen compounds or epichlorhydrin (which yield ether linkages) olefinic aldehydes (which yield ether/acetal linkages), boric acid (which yields boric ester linkages), sulfonamidoaldehydes, etc.
U.S. Pat. No. 4,992,176 which issued Feb. 12, 1991 to Texaco as assignee of Craig R. Bartels is directed to separation of water from organic oxygenates, such as isopropanol, by use of a membrane system including a support layer of polyacrylonitrile bearing a separating layer of poly(vinyl pyridine) which has been cross-linked with an aliphatic polyhalide.
U.S. Pat. No. 4,728,429 to Cabasso et al, U.S. Pat. No. 4,067,805 to Chiang et al, U.S. Pat. No. 4,526,948 to Resnick, U.S. Pat. No. 3,750,735 to Chiang et al, and U.S. Pat. No. 4,690,766 to Linder et al provide additional background.
Additional prior art which may be of interest includes:
Mobility of Spin Probes in Quaternized Poly(4-Vinylpyridine) Membranes, Makino, Hamada, and Iijima, in Polym. J. (Toyko), 19(6), 737-45, 1987.
Effect of Quaternization on the Pervaporation Rate of Water Through Poly(4-Vinylpyridine) Membrane, Hamaya, and Yamada, in Kobunshi Ronbunshu, 34(7), 545-7, 1977.
Preparation of Separation Membranes, Yamamoto, Toi, and Mishima, patent #JP 61/161109 A2, Jul. 21 1986. (Japanese).
Separation of Some Aqueous Amine Solutions by Pervaporation through Poly(4-Vinylpyridine) Membrane Yamada and Hamaya, in Kobunshi Ronbunshu, 39(6), 407-14, 1982.
Complex Formation of Cross-linked Poly(4-Vinylpyridine) Resins with Copper (II), by Nishide, Deguchi, and Tsuchida, in Bulletin of the Chemical Society of Japan, Vol. 49(12), 3498-3501 (1976).
Although many of these membrane systems of the prior art may exhibit satisfactory Flux and Separation, it is found in practice that after the membrane assembly has been in use to effect a particular separation or for an extended period of time, the assembly may tend to deteriorate and become brittle. In the membrane assembly of the above-noted U.S. Pat. No. 4,992,176 for example, it is found that mechanical stability deteriorates to a degree that the Separation undesirable decreases. Although the length of time to reach this undesirable state with may vary depending on the nature of the charge and the conditions of operation, it may occur in less than a few hours or in a few days.
Inspection of the membrane system reveals that deterioration is due to the failure of the adhesion between the separating layer and the support layer. In the case for example of a polyvinyl pyridine separating membrane layer mounted on a polyacrylonitrile support, it is found that the bond therebetween has failed and this is evidenced by the visible separation of the layers as well as by the cracking of the separating membrane layer at those points at which the bond has failed.
It is an object of this invention to provide a membrane system, characterized inter alia by its ability to separate water from an organic oxygenate typified by ethylene glycol, which possesses a high degree of mechanical stability during such separation operations. Other objects will be apparent to those skilled in the art.